A color filter employed for a color liquid crystal display device usually comprises red, green and blue pixels formed on a light-transmissive substrate (e.g., glass plate) and a black matrix (black resin layer arranged in the form of grid) provided on a space area between the pixels to improve contrast of a displayed image. On the pixels and black matrix, a protective layer is generally provided in order to render the surface of the pixels and black matrix even, whereby a covered color filter (color filter substrate) is obtained.
The color liquid crystal display device is essentially composed of a liquid crystal cell comprising a color filter substrate having an electrode, a substrate having an opposite electrode and liquid crystal injected into a gap between these substrates, and a polarizing film.
In more detail, the liquid crystal cell is usually prepared by forming transparent electrodes and an orientation layer on the protective layer of the color filter in order, to prepare an color filter substrate having electrode; bonding the color filter substrate to a substrate having an opposite electrode via spacers; and injecting liquid crystal into a gap between these substrates. Further, if desired, the polarizing film and the optical compensatory sheet are provided on upper and under side surfaces of the liquid crystal cell, to prepare a color liquid crystal display device.
When the evenness of the surface of the protective layer is poor, the distances between the substrates (distances of the gap) becomes uneven along the surface plane because the substrates are combined to each other via both protective layers of the substrates intervening therebetween. The uneven distances of gap lowers display characteristics (e.g., giving uneven displayed image).
Therefore, the unevenness of the surface of the protective layer is generally required to be as little as possible, and preferably in the range of+0.1 .mu.m to-0.1 .mu.m in the case of TFT-LCD (thin film transistor type liquid crystal display), and in the range of+0.05 .mu.m to-0.05 .mu.m in the case of STN-LCD (super twisted nematic type liquid crystal display).
The protective layer has been formed by coating a liquid composition formulated for protective layer on the pixels (and black matrix) by means of a coating method such as gravure coating, and exposing the coated layer to light. Recently, Japanese Patent Provisional Publications No. 1(1989)-262502 and No. 4(1992)-115202 have proposed a process of transferring a protective layer onto the pixels, and a process of transferring a photosensitive resin layer onto the pixels and exposing the transferred photosensitive resin layer.
Examination of the surface of the protective layer in connection with its evenness is, for example, carried out by observing interference of light produced when the surface of the protective layer provided on the pixel sheet is irradiated with a monochromatic beam such as a light from a sodium lamp. Otherwise, the examination is conducted by preparing a liquid crystal display device using a color filter provided with a protective layer and actually driving the display device and then finally observing the conditions (e.g., evenness) of an image displayed on the display device by applying a light from the back sidet.
The examination of the surface of the protective layer provided on the pixel sheet using the sodium lamp is not advantageous, because the pixel sheet which is judged to have uneven protective layer thereon cannot be practically used and should be thrown away. Further, the examination requires a skilled art because the interference on the surface of the protective layer varies depending on directions of light applied onto the surface. The examination on the finally assembled liquid crystal display device has a great risk because the display device which is judged to have uneven liquid crystal layer therein cannot be practically employed and should be thrown away.